1. Field of the Invention
The present invention relates to printing methods and apparatus for registering printed dots and more particularly to such methods and apparatus in which misregistration between dot generating elements is measured and used to shift the position of the printed dots to improve registration.
2. Description of the Related Art
Raster or matrix type printers are capable of printing a plurality of rows of dots on print media, such as paper, as a print cartridge moves relative to the paper. The print cartridge in some such printers comprises a print bar, or in the case of color printers a plurality of print bars, which are as wide as the paper. The only movement is that of the paper relative to the print bar as dots are printed on the paper. In the case of a swath printer, a print carriage containing print cartridges moves laterally above a sheet of paper during a first printing scan while the paper remains stationary. The paper then advances and the print carriage moves across the paper again while the paper remains stationary in its new position.
A color printer includes a plurality of cartridges for printing different colored dots on the same portion of a sheet of paper. If the dots are not correctly registered relative to one another the printed information contains a fuzzy or halo quality which is undesirable.
Prior art techniques exist for registering dots printed by separate cartridges on a single printer. For example, in U.S. Pat. No. 5,289,208 issued Feb. 22, 1994 by Haselby and assigned to the assignee of the present application, an optical sensor detects misalignment of horizontal and vertical lines printed by separate ink-jet cartridges mounted on a print carriage. The Haselby device uses techniques and circuitry to change the timing of nozzle firing and to select different cartridge nozzles for printing dots so as to properly register dots printed by nozzles on different printhead cartridges. In addition, the angular position of one of the cartridges is adjusted relative to one another by adjusting a cam which tilts the cartridge.
U.S. Pat. No. 4,922,270 to Cobbs et al. discloses an inter pen offset determination and compensation in multi-pen thermal ink jet printing systems and is assigned to the assignee of the present application. The Cobbs et al. system uses an aperture plate disposed in the print media plane which prevents an ink drop ejected from a nozzle over the plate from striking the print media. An optical drop detector determines when drops are intercepted by the aperture plate thereby generating nozzle position data. This data is used to adjust timing of drop firing to correct for misalignment between nozzles on different cartridges.
U.S. Pat. No. 4,709,244 to Piatt et al. includes an optical scanner which sense leading and trailing edges of an ink cartridge nozzle plate thereby generating position data, along the axis of cartridge movement, indicative of the relative positions of nozzles in different cartridges. The data is used to adjust drop firing times to improve printed dot registration. Both the Piatt et al. and Cobbs et at. systems incorporate carriage position sensors for determining the position of a carriage upon which the print cartridges are mounted.